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Varnishes
Varnishes are liquid translucent substances which, when applied to a surface in a thin film, dry on exposure to the air, giving a decorative and protective coating.Most varnishes give a glossy finish though some are made to dry with a mat or semi-mat film.
The two main types of varnish which concern the'decorator are oil and spirit varnishes.
Oil varnishes consist of a combination of gums or resins, drying oils, drier, and a solvent or thinner, and harden mainly by the evaporation of the solvent and the oxidation of the oil.
Spirit varnishes consist of solutions of gums or resins in a solvent and dry entirely by the evaporation of the latter.
Oil varnish is by far the more important of the two types and the term varnish, by itself, is invariably taken to imply an oil varnish.
The manufacture of varnish, though superficially a simple operation, is far from being so in practice and requires considerable skill and experience to produce good and uniform results.
The requisite amount of resin or " gum" is first melted or " run " in a special metal pot.
The fumes given off during this process are collected by means of a hood and conveyed through pipes to condensers.
Meanwhile, the oil, which is usually linseed oil, is being heated in a separate vessel to about 4000 or 5000 F. according to the nature of the gum.
The correct proportion of hot oil is then added slowly to the melted resin and the mixture is then cooked for a further period, any necessary driers are added, and then the pot and its contents are removed from the fire, allowed to cool until a temperature of about 3000 F. is reached, when thinners in the form of turpentine or white spirit are added.
The varnish is finally run through a filter press, to remove any impurities, and run into tanks.
As freshly-made varnish proves unsatisfactory in use, it is allowed to mature for some months before it is despatched from the factory.
The gums and resins used in the process are exudations from trees, which harden in the air, usually in an amorphous state.
The difference is that gums are soluble in water, forming a mucilage, and are insoluble in alcohol.
As a rule they are transparent, and are all the products of living trees or bushes.
They are chiefly used in dissolved form as adhesives, but also to add body and a glossy finish to water-colours.
Resins are sometimes transparent, often cloudy, and occasionally opaque in the mass; they are of vegetable origin, but many of them are fossiliferous, the exudation of extinct trees.
Others are derived from existing species.
The chief difference between them and gums is that they are insoluble in water and often only partially soluble in alcohol.
Most of them after being laid on in a diluted form dry with a hard and more or less glossy surface.
They are, therefore, important constituents of polishes, varnishes (clear or opaque), and enamel paints, as well as stoving enamels and lacquers. In the trade the terms are employed rather loosely.
Many so-called gums are really resins.
But there is some excuse for this, as a few of the carbohydrates partake of the nature of both gums and resins.
These are valuable, because they dry with a more glossy and a harder surface than gums,
but with a slightlv less hard, less brittle, and more elastic surface than resIns.
Associated Pages: xxxxx GumssxxxxxxResinsxxxxxxLaquersxxxxxxGold Size